The present invention relates generally to motor control systems and more particularly to a method and apparatus for developing signals representing the value of motor flux through the sensing of motor voltages and currents.
Many control systems for alternating current (ac) motors employ signals which are proportional to motor flux. Examples of this include U.S. Pat. Nos. 4,230,979 "Control Current Inverter and Motor Control System", by P. M. Espelage et al, issued Oct. 28, 1980 and 4,088,934 "Means For Stabilizing A-C Electric Motor Drive System", by J. D. D'Atre et al, issued May 9, 1978. Many other examples could be cited. In some instances, only the total motor flux is required for control. In other instances, the direct and quadrature components of the flux are used, at least in an intermediate sense, for use in developing other control signals such as those representing torque, slip and internal power factor. In this regard, 4,088,934 patent is again cited as an example as is co-pending application Ser. No. 332,180 for "Twelve-Pulse Operation of a Controlled Current Inverter Motor Drive" by J. H. Culter et al, Dec. 18, 1981.
In most instances the development of flux signals does not present a significant problem. Flux sensors can be used, as can additional sensors for motor torque, etc. In certain situations, however, the use of such additional sensors creates serious expense and problems. As an example, in the normal excavating shovel, the motor(s) which are used to propel the equipment are located adjacent the propelling equipment (e.g., treads) while the basic power supply and control therefor are located in the cab which is pivotable with respect to the propelling equipment. As such, all electrical connections to the motor are normally made through slip rings. Such slip rings not only represent an initial extra expense, but their maintenance, in the extremely hostile environment to which excavating shovel is exposed, is considerable. This maintenance problem is particularly pronounced in the low power level situations which exist with low power feedback signals such as would be derived from "flux coils" located in the motor.
One well known method of obtaining a signal representative of motor flux, when so-called flux coils are not used, is to integrate the term, motor terminal volts minus the current times resistance (IR) drop of the motor. While the resistance of a particular motor can be readily measured when the motor is not running, it must be remembered that the motor resistance will vary with temperature and, perhaps, with aging of the motor. Practical, reliable means to measure motor resistance under operating conditions are not readily available.
The need for an accurate indication of motor resistance, if the integrating method of determining motor flux is to be used, is apparent when it is realized that in most controls, if the value used for the resistance is too low, the motor control may limit the load torque to a value less (e.g., to one-half) of that actually permitted. Conversely, if the resistance value used is higher than the actual resistance, not only will the torque be incorrect, but the motor control man rapidly become unstable. The consequences of the latter situation are readily apparent and the difficulties associated with the first becomes so when it is realized, that in many circumstances such as the shovel use earlier indicated, the motor must deliver full rated torque at low speeds.